Thread: newb air/fuel question

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Hi guys,

I'm not too experienced in carbureted engines, I've only worked on newer motors. That said, I purchased a 1975 Firebird that had a GM Fast Burn 385 dropped into it in 2007. This is the motor:
GM Performance Engines | Fast Burn 385 Turn-Key | Chevy Crate Engine | GM Crate Motor | GM Crate Engine

So I have this problem where if I floor it quickly, the engine cuts out. The exhaust smells very rich, so I'm thinking maybe I'm running rich and the engine is drowning when I floor it. I looked up the carb part number and found an instruction manual on it, however I'm not finding how to adjust if you're running rich/lean. I looked straight down into the primaries and I can see the two jets of fuel when I open the throttle. There is a screw directly above the area where the two jets of fuel comes out that faces directly upwards, is this the adjustment for fuel/air ratio?
I'm aware that it might be something else causing the cut-out, but considering how rich is smells and the fact that i get 9.6 mpg, my gut tells me I'm running a bit rich.

Any and all advice is greatly appreciated.

what carb are you using? First things first would be to make sure your fuel pressure is correct, what is the PSI?

It's a Holley 750 CFM (p/n 0-80508S), which you can see here:

Holley Performance Products 750 CFM Four Barrel Street Carburetor*0-80508S


Recommended fuel pressure is 5-7 PSI and I'm running about 5.5.

I would suggest buying the book on tuning a holley carb , you could also find a lot of tuning info on line. I dont remember exactly , I am sure some one else will , it been a while. Check your float level. Then put a vacum gauge on the engine and use the readings to make sure you have the correct power valve, Check the throttle cam. I believe it was vacuum reading divided by 2 and that should be the number for your power valve. Power valve and throttle cam come first to mind for a stumble while hitting the gas. If those are correct the next thing I would check is the spring rate for your secondaries. Any auto parts will have a box of springs specifically for holley that you can use to tune. Really get the little red book, it will simplify it for you , holley carbs are easy to tune once you know what you are doing . Its been a few years for me though.
QUESTION I have a vacuum secondary carburetor that bogs when the secondaries come in. What will cause this?
ANSWER Bogging and hesitation are caused by the secondaries coming in too quickly. You can install a heavier secondary spring and this will prevent the secondary from coming in too soon. If the engine is sluggish in response at full throttle then the secondaries may not be opening soon enough. You will then need to go to a lighter spring. The spring kit is part number 20-13 which will have 7 different springs to fine tune with.

Good advice, I just ordered it from Amazon, thanks. In the end it doesn't really matter because I'm dropping an LS2 in it, but I always want my engines running perfectly.